1. Field of the Invention
The present invention relates to a piezocomposite having a ceramic structure in a fine structure form, to an ultrasonic probe for ultrasonic diagnostic equipment, to ultrasonic diagnostic equipment using the probe, and to a method for producing the piezocomposite.
2. Related Background Art
Conventionally, an attempt has been made to substitute a piezocomposite made of a piezoelectric ceramic and an organic polymer for a piezoelectric ceramic as an ultrasonic transmitting/receiving part in an ultrasonic probe for use in medical ultrasonic diagnostic equipment, with a view to providing a higher resolution and a broader bandwidth for the ultrasonic probe. Among such piezocomposites, a piezoelectric material that has attracted attention (hereinafter referred to as 1-3 piezocomposite) has a structure in which a multiplicity of piezoelectric ceramics 31 in cylindrical or prismatic shapes (for instance, quadratic prism) are arranged in a matrix made of an organic polymer 41 as shown in FIG. 24. This has been found to be useful theoretically, thereby resulting in that production of the same has been attempted. The prior art concerning these 1-3 piezocomposites is disclosed in, for instance, “Cho-onpa Binran (Ultrasonic Wave Handbook)” edited by Cho-onpa Binran Editing Committee, Maruzen, and published on Aug. 30, 1999, pp. 129-133, etc.
Though the usefulness of the 1-3 piezocomposite has been recognized, there have been few commercially available commodities of the 1-3 piezocomposite as actual ultrasonic probes for ultrasonic diagnostic equipments. Two of the main reasons for the same are: (1) that an extremely fine structure is required, and its production is very difficult; and (2) that, even if manufacture is possible, the production cost is very high.
It is considered that the piezocomposite for use in the ultrasonic diagnostic equipment ideally has a configuration in which the piezoelectric ceramics have a diameter and an interval therebetween of approximately 10 to 200 μm each, which is derived from a frequency used and an acoustic impedance of the piezocomposite. On the other hand, it is said that the best transmission/reception efficiency is obtained in the case where a ratio between a diameter and a length (length/diameter, hereinafter referred to as “aspect ratio”) of a prismatic piezoelectric ceramic is set to approximately 5 to 6. In other words, the piezoelectric ceramic in the piezocomposite is required to have a length of approximately 50 μm to 1200 μm.
Considering the foregoing together, the most desirable piezocomposite would have an extremely fine structure in which a plurality of fine piezoelectric ceramics, each of which is in a thin line form with a diameter of several tens μm and an aspect ratio of approximately 6, are provided in an organic polymer matrix, and it is demanded in the market.
As an example of the prior art techniques concerning the method for producing a piezocomposite, a method for producing the 1-3 piezocomposite is proposed by JP 1789409 and JP 1590342 in which a plurality of cut grooves are formed by machining in a piezoelectric ceramic block used as a material, and a resin is impregnated in the cut grooves and cured.
Furthermore, a production method in which laser processing is carried out instead of the conventional cutting technique such as dicing is proposed by JP 5(1993)-33836 B. In the foregoing known example, a laser beam is scanned thereover so that parallel cuts in two directions that cross each other are formed in the piezoelectric ceramic. By so doing, a piezoelectric ceramic structure having a fine structure is provided, and thereafter, a resin is impregnated in void portions and cured.
However, in such methods in which cut grooves are formed directly in a relatively large piezoelectric ceramic block by dicing or laser processing so as to provide an aggregate of fine sintered piezoelectric pieces, there is a drawback in that it is difficult to form a desired shape, or even if it is possible, only a low manufacturing yield is obtained and the production cost increases. With the present machining technique, considering a case where cut grooves with a width of several tens μm are formed in a ceramic block, the yield more rapidly decreases as the aspect ratio increases, or in other words, as the depth of the cut grooves increases. This is because the piezoelectric ceramic tends to be broken more often as the piezoelectric ceramic obtained (remaining) after cutting the cut grooves is thinner. In the case where even only a few of fine sintered piezoelectric pieces in a piezocomposite formed in an intended shape are damaged, this may make the entire piezocomposite defective. Therefore, it is very difficult to produce a piezocomposite having the aforementioned currently required shape by the conventional method in which cut grooves are formed by machining.
Furthermore, since it is difficult to directly process a ceramic by machining such as cutting or laser processing, a plurality of methods have been proposed in which a sintered piezoelectric ceramic having a required structure is formed using a mold having a shape reverse to the required structure, and thereafter, a resin is impregnated in void portions and cured.
For instance, the following method for obtaining a piezocomposite is proposed. Namely, the foregoing mold is formed with a resin, and a ceramic slurry is filled in the resin mold, then, the resin mold is burned out so that a structure made of only a ceramic powder is formed, and further, the ceramic powder is sintered. By so doing, a sintered piezoelectric ceramic having a fine structure, and finally a resin is impregnated in void portions and cured, whereby a piezocomposite is obtained.
However, it is not easy to burn out the resin mold so as to leave only the ceramic powder before sintering. Upon the burning out of the resin mold, the resin flows, thereby breaking the ceramic powder structure before sintering. Furthermore, as the structure is made finer, the difficulty in burning out the resin mold increases.
Therefore, as a method for causing only the resin to disappear without damaging the ceramic powder structure before sintering, JP 2924664 has proposed the heating in vacuum, the laser abrasion, the plasma etching, and the use of a low-viscosity solvent.
Furthermore, as another method, JP 11(1999)-274592 A has proposed a method in which a mold is made of a silicon material that is not burned out when the ceramic is sintered, and after sintering the ceramic structure, only the mold is removed. More specifically, a plurality of fine holes are formed in a silicon material by ion etching, a slurry made of a piezoelectric powder and a binder is applied to the holes and dried so that the binder is removed therefrom, the obtained matter is covered with a protective ceramic powder and is sintered while pressurized, then, the protective ceramic powder is removed after the sintering, and finally, the silicon material is removed by etching. In other words, it is a method in which a mold having a shape reverse to a required structure is made of a material that is not burned out at a temperature at which a ceramic is sintered, and after sintering the ceramic powder structure, the mold is removed by etching.
These methods, however, are complexity, and need complex steps and a long time for making the mold disappear, thereby unavoidably leading to an increase in the production cost. Furthermore, manufacturing devices used therein are expensive. Moreover, in a method utilizing a resin mold having a shape reverse to a required structure, that is, both of the method utilizing a resin mold and the method utilizing a silicon material, the mold has to be made to disappear with certainty. The cost of manufacturing the mold and the cost of raw materials for the mold result in an increase in the cost of manufacturing the piezocomposite as a final product.
Another method is a method in which a plurality of sintered piezoelectric ceramics, each of which is in a rod shape, are prepared beforehand, and they are arranged and integrated, whereby a piezocomposite is produced, which has been proposed by Wallace Arden Smith, in “The Role of Piezocomposites in Ultrasonic Transducers”, Proceedings of the 1989 IEEE Ultrasonic Symposium, pp. 755-766, 1989.
However, the method in which sintered piezoelectric ceramics, each in a rod shape, are prepared beforehand and arranged later on has a drawback in that the handling of the sintered piezoelectric alone is more difficult as the sintered piezoelectric in an object piezocomposite has a finer structure, thereby making the production difficult, or even if the production is possible, decreasing the yield, which results in an increase in the production cost.
As another method, the following method has been disclosed in U.S. Pat. No. 4,514,347 and U.S. Pat. No. 4,572,981. Sintered ceramic plates are laminated and integrated using another material so as to form a block, and a sheet obtained by slicing the block is prepared as a first stratum. Then, the sheets are laminated and integrated again using another material that forms a second stratum, and the lamination is sliced, so that a 1-3 piezocomposite is obtained.
The foregoing method, however, is not capable of avoiding the continuous cutting through a distance longer than the length of sintered piezoelectric thin wires in a piezocomposite as a final object, when the lamination block is cut to obtain the first stratum. Therefore, likewise, there is a drawback in that the production is difficult, or even if it is possible, the yield is not sufficient, which causes the manufacturing cost to increase.
Thus, by the conventional methods that have been proposed or experimented so far, the production of piezocomposite having a fine structure that is demanded currently is difficult, or even if it is possible, it requires a high production cost.
Furthermore, there is another problem in that in the conventional techniques, a material that is impregnated in void portions of a sintered piezoelectric is limited to one kind because of the production method, and consequently, it is difficult to adjust physical properties of an obtained piezocomposite freely.
Considering the foregoing conventional techniques in detail, the inventors of the present invention concluded that in order to obtain a piezocomposite having a shape required in the market currently, it is necessary to realize at the same time the production of extremely fine sintered piezoelectric pieces with a high yield and the arrangement of the produced fine sintered piezoelectric pieces with high precision.